The subject matter described herein relates to circulators and isolators used in RF devices, and more particularly to an integrated circulator or isolator having a packaging configuration suited for use with phased array antenna systems and other RF devices where space and packaging limitations preclude the use of conventional circulators or isolators.
In phased array antennas, radar systems and various other forms of electronic sensor and communications systems or subsystems, ferrite circulators and isolators provide important functions at RF front end circuits of such systems. Typically, such devices, which can be broadly termed “non-reciprocal electromagnetic energy propagation” devices, are used to restrict the flow of electromagnetic wave energy to one direction only to/from an RF transmitter or RF receiver subsystem. Circulators and isolators can also be used for directing transmitting and receiving electromagnetic energies into different channels and as frequency multiplexers for multi-band operation. Other applications involve protecting sensitive electronic devices from performance degradation or from damage by blocking incoming RF energy from entering into a transmitter circuit.
A conventional microstrip circulator device consists of a ferrite substrate with RF transmission lines metalized on the top surface to form three or more ports. A ground plane is typically formed on the backside of the substrate, as illustrated in FIGS. 1 and 2. An isolator is simply a circulator with one of the three ports terminated by a load resistor.
A circulator device uses the gyromagnetic properties of the ferrite material, typically yttrium-iron-garnet (YIG), for its low loss microwave characteristics. The ferrite substrate is biased by an external, static magnetic field from a permanent magnet. The magnetic lines of flux in the ferrite substrate propagate in only one circular direction, thus forming a non-reciprocal path for electromagnetic waves to propagate, as indicated by arrows in FIG. 1. The higher the operating frequencies, however, the stronger the biasing field that is required, which necessitates a stronger magnet.
A phased array antenna is an antenna formed by an array of individual active module elements. In applications involving phased array antennas, each radiating/reception element can use one or more such ferrite circulators or isolators in the antenna module. However, incorporating any device into the already limited space available on most phased array antennas can be an especially challenging task for the antenna designer. The space limitations imposed in phased array antennas is due to the fact that the spacing of the radiating/reception elements of the array is determined in part by the maximum scan angle that the antenna is required to achieve, and in part by the frequency at which the antenna is required to operate. For high performance phased array antennas, this spacing is typically close to one half of the wave length of the electromagnetic waves being radiated or received. For example, a 20 GHz antenna would have a wavelength of about 1.5 cm or 0.6 inch, thus an element spacing of merely 0.75 cm or 0.3 inch. This spacing only gets smaller as the antenna operating frequency increases. Thus, a conventional circulator device (e.g., a conventional microstrip circulator) has physical size constraints in all 3 dimensions due to its having a ferrite substrate with metalized RF transmission lines on the substrate and a permanent magnet attached therewith.
As a consequence, a conventional microstrip circulator/isolator requires mounting on a phased array module circuit board made of a non-magnetic substrate material totally different from that of the ferrite substrate. Complicating matters further, the size of the ferrite circulator/isolator does not scale down as the operating frequency increases because of the need for a stronger permanent magnet with the increasing operating frequency. The need for a stronger permanent magnet is harder to meet due to material constraints. Furthermore, wire bonding connections are required for connecting conventional circulator/isolator ports with the rest of a microwave circuit. Accordingly, the packaging of a conventional circulator/isolator becomes more and more difficult and challenging within phased array antennas as the operating frequency of the antenna increases or its performance requirements (i.e., scan angle requirement) increases. These same packaging limitations are present in other forms of RF devices where there is simply insufficient space to accommodate a conventional circulator or isolator.
Accordingly, circulator/isolator assemblies may find utility in RF communication applications.